Abstract
Surface observations are used to elucidate the deformation mechanisms responsible for the superplastic effect in Ti–6Al–4V. High-temperature in-situ tests for tensile and shear deformation modes are performed in the scanning electron microscope at temperatures in excess of 700∘ C. Grain boundary sliding is predominant; the micro-mechanics of accommodation are consistent with the dislocation-based Rachinger theory. The volume fraction of β plays a crucial role. For temperatures greater than 850 °C, the α grains remain unaffected; cavitation is minimal and slip bands needed for dislocation-based accommodation are detected in the β phase but are absent in α. At this temperature, grain neighbour switching is observed directly under shear deformation. At a temperature lower than 850∘ C, the β volume fraction is lower and a different mechanism is observed: slip bands in α and cavitation are found to accommodate grain boundary sliding. In addition, an increase in the α phase intragranular dislocation activity triggers the formation of subgrains and dynamic recrystallisation, consistent with the Rachinger dislocation creep effect. For temperatures lower than 700∘ C, superplasticity is absent; classical creep behaviour controlled by dislocation climb persists. A numerical treatment is presented which accounts for the Rachinger effect. The computational results are used to deconvolute the contributions of each of the competing mechanisms to the total strain accumulated.
Highlights
The mechanisms of superplasticity [1,2]e as observed in metallic systems based upon titanium, aluminium or iron e are controversial
Why? One reason may be that great emphasis has been placed e at least traditionally e on post-mortem analyses of microstructures at ambient temperature, or else deductions made via the analysis of the temperature- or stress-dependence of the strain rate
Unequivocal proof concerning the precise details of the grain boundary sliding accommodation process e whether it is
Summary
The mechanisms of superplasticity [1,2]e as observed in metallic systems based upon titanium, aluminium or iron e are controversial. One reason may be that great emphasis has been placed e at least traditionally e on post-mortem analyses of microstructures at ambient temperature, or else deductions made via the analysis of the temperature- or stress-dependence of the strain rate This introduces a degree of uncertainty, since one cannot be completely sure of the underlying physical mechanisms which are operative on the critical scales: that of the grain size, the grain boundaries and the dislocations. The former assumes that sliding is accommodated by dislocations in the lattice, whilst the latter that boundary mobility is a consequence of stress-driven diffusion These phenomenological descriptions do not include any details of the necessary accommodation mechanism which allows for continuity, e.g. at grain boundaries and triple points. Most researchers [10,18,19] agree that a range of possible deformation mechanisms is likely, with the dominant one dependent upon temperature, strain rate and microstructural features
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